The present invention generally relates to tracking control systems in magnetic recording and reproducing apparatuses, and more particularly to a system for performing tracking control so that a most suitable tracking state is obtained by detecting the tracking error, in a magnetic recording and reproducing apparatus having a mechanism for swinging a rotary head which performs recording and reproduction with respect to a magnetic tape.
Conventionally, magnetic recording and/or reproducing apparatuses (hereinafter referred to as VTRs) which record and/or reproduce video signals on oblique tracks in a magnetic tape by use of rotary heads, have been in wide practical use. And recently, in order to extend the recording and reproducing time of the VTR for home use, apparatuses have been realized in which the tape travelling speed is reduced to one-third normal speed with respect to a cassette tape for performing two hours of normal recording and reproduction, for example, and using magnetic heads having narrow track widths, to perform six hours of recording and reproduction. In this case, the duration of the recording and reproduction is increased to three times that upon normal recording and reproduction. In the above type of VTR, some degradation is introduced in the signal-to-noise (S/N) ratio during the six-hour recording and reproduction compared to the normal two-hour recording and reproduction. However, the above six-hour recording and reproduction can be performed with sufficient S/N ratio for practical purposes.
When an extended-duration recording or reproduction is performed by reducing the track width, track pitch, and the like, the recording and reproducing operation is easily affected by tracking error as compared to the case of normal recording or reproduction in which the track width and the track pitch are large. Hence, during the above extended-duration recording and reproduction, it is essential that an accurate tracking operation is performed.
However, in reality, the size, angle, and the like of a mechanism in a tape moving system are not completely identical among different magnetic recording and/or reproducing apparatuses, and slight variations exist within the tape moving system. Inconsistency in the size and configuration of a tape guiding groove formed in a fixed guide drum, mounting height and mounting angle of a tape travel guide pole, mounting position and mounting angle of the fixed guide drum, and the like, are the main causes for the above variations introduced in the tape moving system. Accordingly, when a magnetic tape recorded by one VTR is reproduced by another VTR, bends are relatively introduced in the track on the magnetic tape with respect to the scanning loci of the rotary magnetic heads. When this kind of bends are introduced in the track on the magnetic tape, tracking error is introduced during scanning operations of the rotary magnetic heads. Furthermore, when inconsistency exists in each VTR with respect to the length of the magnetic tape along the tape travelling path, from a position where the rotary magnetic head begins to make contact with the magnetic tape to where a control head for recording and reproducing a control signal with respect to the magnetic tape is positioned, the above tracking error is also introduced. Reproduction cannot be performed with a fine S/N ratio when the above described tracking error exists.
Conventionally, as a system for correcting and controlling the above tracking error, there was a system in which a head moving mechanism is used to vary the height positions of the rotary magnetic heads by a control signal. In this system, a signal having a constant frequency of 480 Hz, for example, is applied to the head moving mechanism so that the heads swing with a small amplitude during each track scanning period. The quantity and direction of the tracking error are detected from the variation in the level of the reproduced signal. Hence, the control signal is obtained from the above detected result, and the control signal thus obtained is fed back to the head moving mechanism.
However, in the above conventional system, there was a disadvantage in that the level variation in the reproduced signal is large, since the heads swing several times during each track scanning operation of the heads. Moreover, when this system is applied to a so-called azimuth recording and/or reproducing apparatus in which recording and/or reproduction is performed by use of a pair of heads having gaps with azimuth angles in mutually opposite directions, color unevenness is introduced due to deviation introduced in the time base by the swinging of the heads, since the gaps in the heads form certain angles with respect to the swinging directions of the heads. In addition, the tracking error is constantly corrected during a scanning operation with respect to one track. Therefore, when the response of the above head moving mechanism is slow, the correction of the tracking error cannot be performed accurately, and in some cases, the tracking error is increased instead.
The bends in the track do not differ for each track, and the bends are commonly introduced in each of the adjacent tracks due to the various inconsistencies in the tape moving system among different recording and/or reproducing apparatuses. That is, a track bend identical to that existing at a part of the tape which is a certain distance from the edge of the tape with respect to one track, is introduced along the longitudinal direction of the tape for substantially the entire length of the tape, at parts which are the above certain distance from the edge of the tape.
Accordingly, a tracking control system was proposed in a U.S. patent application Ser. No. 225,006, now U.S. Pat. No. 4,402,022, filed on Jan. 14, 1981 entitled "TRACKING CONTROL SYSTEM IN A MAGNETIC REPRODUCING APPARATUS" in which the assignee is the same as that of the present application. This proposed tracking control system comprises head swinging means responsive to a sinusoidal wave head swinging voltage for varying the height position of the rotary magnetic head, and swinging the scanning position of the rotary magnetic head with respect to the track of the magnetic tape in the width direction of the track in a sinusoidal manner, one or a plurality of memory means for sampling and memorizing a voltage which is to be supplied to the head swinging means, at a point in time corresponding to when the rotary magnetic head passes one or a plurality of predetermined relative tracing positions with respect to the magnetic tape, level detecting means for detecting the level of a signal reproduced by the rotary magnetic head at a point in time identical to that of the above, maximum level detecting means for detecting whether the level detected by the level detecting means has reached a maximum, controlling means for enabling the read-out of the voltage memorized in the memory means upon detection of the maximum level by the maximum level detecting means, and adding means for adding the voltage read out from the memory means and the head swinging voltage supplied from the head swinging voltage generating means, and supplying the added voltage to the head swinging means.
However, in the above proposed system, the head swinging voltage is a sinusoidal wave, and the heads are swung so as to depict loci of sinusoidal forms. Hence, at the maximum and minimum points in the sinusoidal wave, the swinging deviation quantities of the heads with respect to a reference position become maximum, and the sensitivity of the tracking error detection becomes high. However, at intermediate parts of the sinusoidal wave between the maximum and minimum points, the swinging deviation quantities of the heads with respect to the reference position become small. Accordingly, there is a disadvantage in that the sensitivity of the tracking error detection becomes low. Moreover, there was a further disadvantage in that the sensitivity of the tracking error detection constantly varies due to the swnging of the heads.
Furthermore, there was a disadvantage in that color unevenness is introduced due to the displacement of the heads although the unevenness is not as significant as in the conventional system, since the heads constantly swing when the heads are scanning a track.